This protocol will investigate the effects of a selective dopamine D1 against (ABT-431) in patients with Parkinson's disease. Patients with Parkinson's disease and a fluctuating response to levodopa will receive single daily infusions of ABT in various concentrations or a placebo on six consecutive days, under double-blind conditions. Parkinsonism will be scored with a clinical coring system (Unified Parkinson's Disease Rating Scale) and blood collected for pharmacokinetic studies of ABT.